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Kangaroos with horn-like protrusions are one of several new species of megafauna discovered in a vast underground cache of fossils under the Australia's Nullarbor desert.

The digs, some begun in 2002 and others recently, uncovered five new species of kangaroo, and the first complete intact skeleton of the marsupial lion, Thylacoleo. The digs were co-ordinated by the Western Australian Museum in Perth.

"Its an excellent discovery," said Dr Gavin Prideaux, a palaeontologist at Flinders University in Adelaide, who is an expert in kangaroos.

"Our knowledge is best for younger sites, and it is rare to find new things from the last two million years - let alone finding five new species of kangaroos. It's a bit of a 'trip' really, especially if one or two are really weird," he quipped.

Samples collected for radiometric dating will be tested over the next few months to determine the age of the sites. They are currently estimated to be between 200,000 and 500,000 years old. Situated in caves beneath the Nullarbor Plain, a dry limestone plateau stretching 750 km across Australia's southwest coast and bordered on the north by the Great Victoria Desert.

The team reached the sites by dropping into the caves using ropes, according to the museum's Dr John Long, although some are crawl-in caves.

The most recent expedition ended early a fortnight ago because of impending rain, Long told ABC Science Online. "If it rains the tracks get very muddy. We had been there two and a half weeks and we were almost finished. We left because otherwise we could have been stuck there."

'Horny' kangaroos

The findings included the skulls of kangaroos with protrusions, and the group initially referred to them as horns. "The first time we looked we asked what the hell are these things?" Prideaux told ABC Science Online via telephone from another dig in Naracoorte, South Australia.

Horns are mostly used by animals in male sexual displays. The group soon realised the projections on the newly-discovered skulls were not horns, but more analogous to brow ridges. "In profile they don't stick up at all," Prideaux said. "These projections are more oriented to protect the eye. They are very weird features about a skull or skeleton."

He suggested that the protrusions may be to protect the eyes of the kangaroos as they burrow into the ground, or as they push into spiky bush structures. "They also have long claws on their hands," Prideaux said, which again could be used for grabbing clumps of spiky bush or for burrowing. "There are lots of aspects we haven't studied yet."

The ancient kangaroos were about the size of a small grey kangaroo, which does not preclude them from digging burrows but does make it more likely they would have been digging for food. "They probably didn't dig extensive systems," he said.

It is likely the kangaroos lived in several parts of ancient Australia. They have very distinctive high-crowned front incisors with a thick layer of enamel, a standard adaptation to abrasive food. Skeletons with the same distinctive teeth have also been found on a site on the York Peninsula and in northern South Australia, explained Prideaux.

But it is very obvious that the kangaroo thrived in the Nullarbor area. "We can see just from the general population of the species that they are pretty abundant in the area. They made up 50% of any skeletal remains," he said.

The new kangaroo species are yet to be named. The taxonomic identification will happen over the next year or two, said Prideaux, as his involvement is basically voluntary - Prideaux went on the Nullarbor dig in his own time.

"If you work as a palaeontologist, you don't necessarily get paid other than your research costs and field costs," he said.